Martha Marcy May Marlene was a disturbing film about a woman escaping from an abusive cult in the Catskill mountains of New York. It earned director and writer Sean Durkin a lot of kudos when it appeared in 2011 – and it speaks volumes about the healthy state of TV drama that Durkin decided to follow it up with a four-part drama for Channel 4 set in Kent (albeit a fictional version of the county). "It was the script first and foremost," the 31-year-old New Yorker explains, in a sleepy drawl that is not unlike the pace of his work. Southcliffe is from the pen of Tony Grisoni, who adapted Red Riding – a story of dodgy coppers and organised crime set against the backdrop of the Yorkshire Ripper – for Channel 4 in 2009.

Over coffee, the American and the Brit discuss the unusual drama they've created. Southcliffe is cinematic, harrowing and ambitious: its four episodes tell the story of a small-town spree shooting, following the killer in the buildup to the crimes, then panning out on the aftermath – a day later, a week later, a year later. At times, it is difficult to watch. In less capable hands, a fictionalised shooting might be used as a cheap dramatic device, transforming memories of shocking events, such as the Raoul Moat shootings in 2010, into headline-grabbing TV. But Durkin has a sensitive eye. The result is subtle menace laced with sadness in a drama that is calm and unhurried – which only makes its tragedy more potent. It's a sprawling ensemble cast, but with Eddie Marsan (a father), Shirley Henderson (his wife), Sean Harris (the killer) and Rory Kinnear (TV reporter) in starring roles.

Grisoni, who grew up in Bournemouth, has form in scripts that blur the line between fiction and reality. As well as Red Riding, he worked on Michael Winterbottom's 2002 people-smuggling docudrama In This World, about two Afghan refugees seeking a better life in Britain. "One thing that made In This World was the fact that what happened in the film was taken from real accounts of people who were smuggled into the UK," he recalls. "That was a turning point – in having to step into someone else's world and be informed by their experiences."

He denies Southcliffe's central conceit is exploitative. "It's not really a story of a spree shooting. It's a story of people who are suddenly robbed of someone very close to them." But, by focusing the first episode on the life of the killer, an outcast who cares for his ailing elderly mother and who is treated unkindly by the rest of the town, you could argue that the audience is being manipulated into sympathising with him, understanding his motivation. Grisoni disagrees, insisting that it's vitally important not to portray the shooter as a cartoon monster. "You must treat him as a human being. He is a human being. He's not a different species. Southcliffe is full of people, as in reality, asking, 'Why did this happen?' Of course you want to know. We couldn't provide the answer."

Only after the bloodshed does Southcliffe reveal its true purpose: it turns out to be less an examination of why spree shootings happen, and more a portrait of grief and devastating aftershocks. "I'd never read or seen anything that I felt could capture loss in this way," says Durkin. "When I read the script, I remember thinking, 'OK, if there was no shooting in this script, you could still have a show about these characters, living the lives they're living.' For me, that was everything. The plot doesn't matter."

Durkin joins Jane Campion, Steven Soderbergh and David Fincher in turning to TV. "I think a lot of television now is able to do much greater things than film," he says when asked why he followed Martha Marcy May Marlene with this. "You know, the idea of a four-hour drama is exciting and different. The things I find really inspiring right now tend to be television."

Grisoni feels the same way. He talks about Ken Loach's 1966 film Cathy Come Home – television so powerful it ended up being discussed in parliament. "It was dangerous. It threatened the status quo," he says, clearly enlivened by the prospect. For a while, he thinks, "TV got soft and boring." But now we're back to a state of "asking difficult questions". That's what makes Southcliffe so tough to sit through, even over the course of a few days. But it is remarkable television. As I get up to leave, Durkin asks me if I could have watched all four episodes in one sitting. No, I say, I don't think I could have.